Quantum chemical study of structure and stability of all 14 isomers of isocytosine

Abstract

Results of quantum chemical computations based on the HF 6–31 G(d,p) (pointwise MP2/HF/6–31G(d,p)) approximation of fully optimized structures, electronic energies, rotational constants, electric dipole moment components and electric field gradient principal values at nitrogen atom sites of all 14 isomers of isocytosine are reported. Together with results of the computation of fully optimized total electronic energies of all 2- and 6-amino- and all 2- and 6-hydroxy-pyrimidine isomers at the same level of approximation, the data are analyzed for relative stabilities of tautomers and conversion energies of geometric isomers with imino and hydroxy substituents. From these quantities and analogous data for all cytosine isomers, an extended system of attractive and repulsive contributions (“increments”) is reported, from which geometric isomer conversion energies may additively be reproduced (within 0.5 kcal mol −1). Two exceptions to this linear additive model are listed. The increments are used to derive estimates for conversion energies of isocytosine and cytosine tautomers. They furthermore form a basis for interpretation of predicted non-planar structures found earlier for two cytosine isomers, for the geometry of non-planar amino groups of some cytosine and isocytosine isomers and for relaxation of local structural parameters upon interconversion of geometric isomers. A number of predictions for rotational spectra of isocytosines and some correlations of electric field gradient tensors at sites of nitrogen nuclei (with sp 2 or p z lone electron pairs) and of quadrupole coupling constants with attractive and repulsive interactions are put forward. A statistics based comparison shows predicted and crystallographic structural parameters of two isocytosine isomers to be significantly different.